jomaxx - 11:00 pm on Feb 16, 2010 (gmt 0)
It may reduce content theft, but like HuskyPup I am skeptical it will "improve" your AdSense stats. It may also cost you link pop if you start banning countries where you've achieved popularity.
Besides, if I were Google, I would penalize sites that do this. They serve the entire world, not just the USA, and it's a major annoyance for their users when the SERP listings are inaccurate or unusable.